how do I lock my screen on my iphone xs while on a call?

I recently purchased an iPhone Xs. Of course this comes with iOS 12. I shelved my iPhone SE still running iOS 10.

On my old phone I could make a call and lock the screen so I could put the phone back in my pocket and no effect the call, be it hanging up, muting, or whatever. I never hold the phone to my head, I always put the phone back in my pocket. I always use either the Apple earbuds or my blue tooth headphones.

So my question is why is it, when I make a call and lock the screen, as soon as something bumps the screen it opens up to the phone call.

There has to be a way around this. This is completely ridiculous.